The present invention relates to a storage system provided with storage devices of a plurality of types having different performances.
Conventionally, a hierarchical storage system has been known. The hierarchical storage system is provided with storage devices of a plurality of types having different performances, and can carry out a data management by using a plurality of layers in the case in which an aggregation of storage areas on the storage devices of the same type is a layer. For instance, the hierarchical storage system can decide a layer that is a data storage location based on the characteristics of the data (such as a frequency of an access). The hierarchical storage system can carry out an efficient data management by storing data to a layer suitable for the characteristics of the data. Patent Citations 1 to 3 disclose the techniques related to the hierarchical storage system.
[Patent Citation 1]
    Japanese Patent Application Laid-Open Publication No. 2007-102455[Patent document 2]    Japanese Patent Application Laid-Open Publication No. 2007-280089[Patent document 3]    Japanese Patent Application Laid-Open Publication No. 2008-90741
In the case in which the data characteristics that decide a data storage location vary as time goes on, the hierarchical storage system can change a layer of a data storage location by carrying out a migration of data. As a method for changing a layer, a method for carrying out in a unit of a logical volume (hereafter referred to as a volume unit change method) and a method for carrying out in a unit of a file (hereafter referred to as a file unit change method) are known.
For the volume unit change method, it is judged whether the data characteristics of data in the logical volume conform to the prescribed conditions or not for every logical volume. In the case in which the data characteristics of data in the logical volume conform to the prescribed conditions, all data in the logical volume are migrated to a logical volume in a layer different from the logical volume.
The volume unit change method is disadvantageous in that a unit of data to be migrated is too large. The too large unit of data is disadvantageous by the following reasons.
That is, data that has been stored into a logical volume (hereafter referred to as volume data) is an aggregation of a plurality of data that has been stored into each of a plurality of data blocks that configure the logical volume (hereafter referred to as block data). The data characteristics of the block data differ from each other for every block data. Consequently, the data characteristics can be variable for one volume data in some cases. In this case, the volume data that has been migrated includes block data to be changed to a layer that conforms to the data characteristics and block data to be changed to a layer that does not conform to the data characteristics. As a data unit is larger, the number of block data to be changed to a layer that does not conform to the data characteristics is more likely to be larger. More specifically, as a data unit is larger, it is difficult to change to a suitable layer that conforms to the data characteristics.
On the other hand, for the file unit change method, it is judged whether the data characteristics of data that configures the file conform to the prescribed conditions or not for every file. In the case in which the data characteristics conform to the prescribed conditions, data that configures the file is migrated to a storage location in a layer different from a storage location that has stored the data.
For the file unit change method, the above described problems in the volume unit change method do not occur. This is because data characteristics related to data that configures a file are the characteristics of the file by ordinary, and the data characteristics are not varied for data that configures a file.
However, the file unit change method is disadvantageous in that the processing for changing a layer requires a great deal of time. This is because the processing that is carried out by a file system that manages a file (such as a retrieval of a directory) requires a great deal of time.